Acoustic measurements have been used in wireline borehole logging for the past four decades. The first wireline acoustic instruments or "tools" were single transmitter and receiver devices which were used to measure the velocity of the first arrival component of an acoustic wave pulse transmitted through the penetrated formation. This component was usually the compressional or "p" wave component. The velocity measurement, or more precisely the travel time of the wave component from the transmitter to the receiver, was used to compute formation porosity in formation evaluation applications. In addition, early acoustic logs were used in the conversion seismic data, initially measured in the time domain, into the depth domain thereby yielding cross sectional displays of geological structure used in the industry as a guide to exploration and development drilling.
During the late 1960s and early 1970s, acoustic wireline devices became more complex and also yielded additional information. In the area of formation evaluation, multiple transmitters and receivers were introduced to reduce the adverse effects of the borehole upon the formation acoustic measurements. In the late 1970s, as the transmission rates of wireline telemetry systems increased, the full wave form of the received signal, rather than just the first arrival time, was measured at a plurality of receivers spaced axially along the primary axis of the logging tool. The analog signals were digitized downhole and digitized wave forms were transmitted to the surface for processing. Processing involved the extraction of the travel times of the compressional and shear components, as well as various tube wave components. In addition, the amplitudes of the various wave train components were determined. In formation evaluation, the full wave form information was used to obtain a more accurate and precise measure of formation "acoustic" porosity. In addition, mechanical properties of the formation were determined by combining amplitudes of the various components of the measured acoustic wave form. This information was used to optimize subsequent drilling programs within the area, to aid in the design of hydraulic fracturing programs for the drilled well, and to greatly increase the accuracy and precision of the conversion of area seismic data from the time into the depth domain.
During this same time period, multiple types of logging sensors were beginning to be run in combination, and the measurements from the various types of sensors were combined to obtain formation evaluation information which exceeded the sum of information obtained from the response of each sensor. As an example, thermal neutron porosity sensors, scattered gamma ray sensors, and acoustic sensors were run in combination. Each sensor yielded an indication of formation porosity. By combining the responses of the three types of sensors, a more precise and accurate measure of porosity was obtained. In addition, information concerning the lithology of the formation was obtained which could not be obtained from the responses of any of the individual sensors.
Much effort in the design of acoustic wireline logging tools was, and today still is, directed toward the minimization of acoustic energy transmitted directly through the body of the downhole instrument. The arrival of this energy component at the receiver or receivers usually occurs before the arrival of energy whose path traverses the formation and the borehole. The travel path is more direct and therefore shorter. In addition, the body of the tool is usually metallic and exhibits a faster acoustic travel time than the formation and the borehole. Since the latter arrivals contain parametric information of interest, the former is considered to be interference or "noise". This direct component is reduced and/or delayed by using a variety of techniques. The component is reduced by acoustically isolating transmitters and receivers from the tool body as much as possible. The arrival of this component is delayed, preferably until after the arrival of components from the formation and borehole, by increasing the effective travel path by cutting a series of alternating slots in the metallic tool body between the transmitter and receiver arrays. This portion of the tool body is commonly referred to as the isolation subsection or "isolator sub". In addition, various mathematical techniques have been used in the processing of full wave form data to remove the direct component of the received wave form.
In addition to noise generated by the direct transmission of acoustic energy through the wireline tool body, additional acoustic noise is generated as the tool is conveyed along the borehole wall. This noise is commonly referred to as "road noise". The adverse effects of road noise are minimized using mechanical and mathematical techniques. The prior art teaches the use of many types of roller mechanical devices whereby the wireline tool is "rolled" rather than "dragged" along the borehole wall thereby reducing the magnitude of the road noise. In addition, since road noise is essentially incoherent, various mathematical methods are used in the processing of full wave form data to greatly reduce the effects of road noise.
The previous discussions have been directed to wireline type measurements wherein the measurements are usually made after the borehole has been drilled. In some drilling operations, wireline logs are made intermittently during the drilling operation, but such logging usually requires that the drill string be removed from the borehole prior to logging. Logging after completion of the drilling operation often reveals that the target formation or formations have been missed by perhaps either drilling too shallow or too deep. In addition, unexpected zones, such as high pressure formations or salt zones, can be encountered during, and adversely affect, the drilling operation. Such encounters can be quite costly and can be fully analyzed with wireline logging only after the encounter. Intermediate logging is likewise costly in that the drilling operation must cease during logging operations. Furthermore, the time interval between the termination of drilling and wireline logging allows the drilling fluid to penetrate or "invade" the near borehole formation thereby possibly introducing error in wireline log measurements. The adverse effects of invasion poses a particularly serious problem for wireline logs with relatively shallow depths of investigation such as most nuclear logs. Possible damage to the borehole can occur during logging and costly drilling rig time and logging equipment time is wasted during stand-by periods for each operation.
Many of the problems discussed above can be overcome by measuring various formation evaluation and other parameters during the actual borehole drilling operation. This is particularly true with acoustic measurements since they not only represent a key formation evaluation measurement but also represent a key seismic tie-in measurement. The problems associated with intermittent logging are essentially eliminated. The need for wireline logging after the drilling can also be eliminated in some cases. Formation evaluation type measurements-while-drilling (MWD) logs can indicate to the driller, in real time, when anomalies such as a fault planes or formation lenses are being penetrated. This is particularly true if the MWD device has a relatively large depth of investigation and if the sensor can make measurements ahead of the drill bit. Such measurements can also indicate to the driller that high pressure formations or salt zones are being penetrated thereby allowing time for remedial steps such as adjusting the weight and salinity of the drilling fluid to be made before these zones adversely affect the drilling operation. Real time measures of drilling dynamics data provide the driller with information concerning the efficiency of the drilling operation. Furthermore, borehole directional information combined with real-time formation evaluation parameters, offset wireline log data and possibly seismic data can be extremely useful in assisting the driller in reaching the targeted zone of interest. The MWD acoustic measurement meets, or contributed substantially to, all of the above criteria as will be discussed in following sections of this disclosure.
The economic, technical, operational and safety advantages of measuring geophysical parameters as well as drilling management parameters, during the actually drilling of the borehole, were recognized in the early 1950's. Commercial measurements-while-drilling (MWD) became available in the late 1970's and early 1980's. These measurements included directional information and a limited number of formation evaluation type services. Additional sensors and services have been added during the intervening time period. In many respects, the sophistication of the sensors are comparable to their wireline counterparts in spite of the harsh environment experienced in using such sensors in the drilling environment. It is feasible, at least in principle, to utilize multiple sensor combination measurement methods developed for wireline tools to obtain new and improved parametric measurements while drilling. Furthermore, it is feasible, in principle, to utilize additional sensors responding to drilling related parameters simultaneously with formation evaluation type sensors. In practice, however, several major problems exist as will be summarized in the following paragraphs.
Wireline acoustic technology has been particularly difficult to adapt to MWD applications. In addition to road noise generated by the drilling assembly dragging against the wall of the borehole, there is an additional source of noise generated by the rotation of the drill bit and the drill string. Further, the slotted isolation sub technique used to isolate transmitters and receivers in wireline applications can not be used in MWD applications in that such slots would mechanically weaken the MWD acoustic subassembly to the failing point. In addition, the previously described full wave wireline acoustic measurement generates tremendous amounts of digital data. These data exceed the telemetry rates and storage capacities of current MWD systems thereby eliminating the option of processing full wave acoustic data at the surface. This problem is compounded when other types of sensors, comparable in sophistication to corresponding wireline applications, are run in combination with full wave acoustic devices. As an example, it is not feasible using current MWD telemetry capacity to transmit simultaneously a plurality of full acoustic wave forms or gamma ray energy spectra or electromagnetic wave attenuation and phase shift data, or a combination thereof, to the surface for processing to determine parameters of interest at depth intervals sufficient to obtain the required vertical resolution of the penetrated formations. The simultaneous transmission of drilling management sensor information such as directional information, weight on the drill bit, and other non formation evaluation type measurements still further overloads current MWD telemetry transmission rates which are of the order of 2 to 60 bits per second. Furthermore, it is not feasible to store copious amounts of raw data downhole sensor data for subsequent retrieval and processing due to relatively limited storage capacity of current MWD systems. Acoustic and other MWD device used for making multiple formation and borehole evaluation type parametric determinations comparable to current wireline measurements require the computation of the desired parameters downhole, and the transmission of the computed parameters of interest to the surface. By using downhole computational and methods, the transmission requirements are reduced by orders of magnitude in that only "answers" are telemetered rather than raw data. This type of downhole computation is also applicable to other types of non formation evaluation type measurements such as signals indicative of the operational characteristics of the downhole equipment as well as measurements indicative of drilling direction and efficiency.